Planning and optimization of mobile telecommunication networks results in the need for propagation models that accurately characterize the propagation of radio frequency signals in a given environment. Predictions of radio frequency signal, or radio wave, propagation are used to estimate quantities such as coverage, serving areas, interference, and so forth. These quantities, in turn, are used to arrive at equipment settings, for example, channel assignments, whose goal is to optimize capacity without sacrificing the quality of the network. Accordingly, it is highly desirable to employ a propagation model that is as accurate and reliable as possible, given the geographical data used as an input to the propagation model.
A conventional approach to propagation modelling is to employ a basic analytic model designed to determine the power received by a mobile station in terms of the power transmitted by a base station, the base station antenna gain, and the mobile station antenna gain. Once the transmitted power and the antennas are selected, the propagation model reduces to evaluating the path loss of the radio frequency signal. Thus, it is highly desirable to compute the path loss as accurately as possible. The path loss is dependent of the distance from transmitting the base station, therefore, the cell radius is one of the required input parameters when calculating the path loss.
In general, path loss is the decrease, or attenuation, of the power of a signal usually occurring as a result of absorption, reflection, diffusion, scattering, diffraction, or dispersion, from an original level. In a mobile communication network, path loss may be determined from several components. For example, path loss may be a combination of distance dependent path loss, path loss due to terrain obstacles, path gain (or loss) due to sloping terrain, path gain caused by over-water propagation enhancement, path loss due to rain attenuation, and/or path loss due to street orientation relative to the propagation path.
A site is herein in this specification defined as the physical location of the base station. A site has several antennas attached to it. The network can see these antennas as one entity or several entities. This means that a signal is split between different antennas or different signals are transmitted to different antennas. The entity or entities are defined as cells and the antennas serving the same signal are defined as a sector. Usually a site has 3 sectors with 2 cells per sector for GSM and 1 cell per frequency for CDMA related networks. The cell radius is the distance from the base station to a point where a signal transmitted from the base station is estimated to be received just below a predefined signal strength. FIG. 1 is a view from a cell planning tool showing the signal strength of GSM and WCDMA 3 sector sites, respectively. It should however be noted that the area covered by the cell defined by cell radius does not determine the area served by the cell. Thus, a mobile terminal located within the cell may be served by another cell.
Currently when a user wants to calculate the path loss by using a cell planning tool the user determines arbitrarily a global cell radius. (An alternative to the global cell radius, is to let the user manually set a cell radius for each cell. A network of one operator may comprise more than 5000 cells, thus the manual cell radius setting would be too time consuming.) That means that the path loss is calculated for pixels within the area defined by this cell radius. This global cell radius is normally determined to be quite large so the user can be sure that no important signals are ignored due to a too small selected distance.
Since every cell has a unique path loss signature, some cells will match the user-defined global cell radius while some other will have a much shorter cell radius than the determined global cell radius. These other cells should therefore have a much shorter cell radius compared to, the global cell radius. The cell radius that should be used is called the actual cell radius.
As mentioned above, the cell planning tool performs path loss calculations up to this user defined distance (i.e. the global radius) from the base station, which implies that the performed calculations for pixels at a distance that are further from the base station than the actual radius are not required. The time required for calculating the path loss has a quadratic relationship to the distance. For example, if it takes 40 minutes to calculate a 30 km cell radius it only takes 10 minutes to calculate 15 km cell radius.
Thus it would be desirable to obtain a method and arrangements for providing an estimated cell radius that is close to the actual cell radius.
U.S. Pat. No. 6,173,186 discloses a method for estimating coverage of a cell. U.S. Pat. No. 6,173,186 provides a cell radius estimation method using path loss and signal source distance data. The method determines the path loss and ranges data from a plurality of predetermined locations within said cell and applies a fixed gradient line fit to the data to obtain cell radius estimates and to determine models for one or more geomorphology classifications. A drawback with this method is that the estimation requires access to a map, denoted geomorphology data. Loading the geomorphology data into the estimation processor requires a significant amount of time.